The fetus' right to life is completely dependent, for survival, on protection and life sustaining nutrition and oxygen provided by it's mother's womb. Without that, it's life would be snuffed out. I wouldn't say that this means the mother's rights are secondary by definition, but as a consequence of the realities of human life. I suppose one could consider this 'secondary' right subordinate for the good of our race and our mutual survival. I consider it mute, that the distinction doesn't matter. It's a simply physical reality in the cycle of human life.
The right to a kidney from some who's life isn't directly dependent of any one individual. That which isn't part of that special and unique relationship between a mother and her unborn child or any other special relationship That would be confiscatory. It is, ofc, tragic, if one cannot acquire a kidney from some kind donor. Such is our lot, based on our own nature (not being willing to make such sacrifices). On the plus side, advancements is medical science have made dialysis more viable for longer periods of time, extending the opportunity for someone in need of a kidney to find one. We live strange lives where we want to do good, but fail to do so. It's like we have forgotten what even the ancient Greeks knew, that the virtues are essential to a just society.